Fluoride ion batteries are one type of primary batteries and secondary batteries, and various researches thereon have been carried out. In fluoride ion batteries, electrolytes (electrolyte solutions) which can conduct fluoride ions are used. For example, Patent Literature 1 discloses an electrolyte solution in which fluoride salts are dissolved in a solvent. Patent Literature 2 discloses a fluoride ion conducting solid electrolyte. Patent Literature 3 discloses an electrolyte solution in which fluoride salts are dissolved in different kinds of organic solvents.